The invention relates to a control arrangement for a shutoff valve actuatable by negative pressure and intended for a negative pressure wastewater system, including a first valve that is actuatable by hydrostatic pressure resulting from accumulated water and that closes or opens a connection that carries negative pressure, a chamber which is pressure-adjustable via the first valve and in which or adjacent to which a main piston preferably acted upon by a spring is displaceably disposed, by means of which piston a negative pressure connection to at least the shutoff valve is controllable as a function of the pressure prevailing in the chamber. At least one second valve is provided, independent of the main piston but positively coupled to it preferably via a driver, by way of this second valve the shutoff valve can be connected upon by negative pressure.
In order to keep bodies of water clean, the wastewater must reach sewage treatment plants. Often, however, this is not possible, either because of disproportionately high costs for conventional sewer systems or because of problematic local conditions, such as the lack of a natural slope, low housing density and unfavorable subsoil, or the fact that the sewer system would have to pass through a groundwater protection area. Even for such problem cases, however, the possibility exists of undertaking sewage treatment, whenever negative pressure drainage or a "vacuum sewer system" is employed.
A corresponding vacuum sewer system includes as its essential components home connection shafts with a control arrangement operating shutoff or aspiration valves without electric current, with an adjoining pipeline system with systematically disposed high and low points, and a vacuum station with wastewater collecting tanks, wastewater pumps, vacuum pumps, and measurement and control systems.
The wastewater first flows out of buildings via conventional gravity drains to shafts, which for example are located at the boundary of premises, and in which the exclusively pneumatically controlled shutoff valves and the associated control arrangement are accommodated.
By means of the mechanism present in the control arrangement, the shutoff valve is opened in the presence of a predetermined hydrostatic pressure, and the wastewater is aspirated into the vacuum line. The valve closes in time-dependent fashion after a few seconds, by spring force and vacuum.
The wastewater itself collects at the low points in the pipeline system and is gradually pushed by incoming air across the ensuing high points in the direction of the vacuum station. From the collecting tank in the vacuum station, the wastewater is then pumped to the sewage treatment plant with conventional wastewater pumps, via a pressure or gravity sewer.
The control arrangement associated with the shutoff valve is intended to enable automatic adaptation both to the batches of wastewater to be aspirated, and to the operating conditions in the drainage pipeline system.
To enable timing control via the pressure-adjustable chamber in a control arrangement known by the trade name "AIRVAC", bores of small diameter, which can easily become plugged, are necessary and the result is that functioning is no longer assured. Moreover, an unequivocal open/closed position of the second valve, transmitting the negative pressure to the shutoff valve, does not exist. This means that the quantity of wastewater or of the mixture of wastewater and air per opening stroke of the shutoff valve is not unequivocally defined. Particularly when there is a large amount of wastewater, this can cause disruptions to operation. It is also disadvantageous that the aspiration time is dependent on the existing negative pressure, in a way which is unfavorable to the overall system, since the opening times in turn are dependent on the prevailing negative pressure. Thus at pronounced negative pressure, the opening time is longer than at slight negative pressure. As an unfavorable result, when the negative pressure is slight less air is aspirated than when the negative pressure is pronounced, even though what would be desired is the opposite.
It is also disadvantageous that an opening of the second valve that enables the negative pressure to reach the shutoff valve can occur at even a slight negative pressure, which nevertheless is inadequate for the aspiration. As a result, the danger is increased that wastewater can be lifted into the frost zone of the pipeline and freeze there.
In order to enable reception of large quantities of water or to enable restarting plants whose operation had been interrupted for a relatively long period of time, it is highly advantageous if the wastewater is aspirated in batches, and if air is aspirated into the negative pressure wastewater system via a valve after each batch. This provides the advantage that if there is a large amount of water or if the storage spaces are nearly overfilled, large columns of water will not be produced in the pipeline system, which could otherwise hinder transport of water.
A control arrangement of the type referred to at the outset can be learned from German Patent Disclosure DE 37 27 661 A1. In order to assure precise adjustment and reliable function of the control device, not only a first valve actuated by a hydrostatic pressure and a structurally complicated timing control acting by way of volumetric change rather than pressure change, but also at least one control valve and optionally at least one minimum negative pressure valve are necessary. Because of the complex mechanical structure specifically of the timing control device, which includes among other elements a diaphragm piston with a hollow protrusion that is guided in a guide bush and which also includes a bracket that acts in turn upon a pivotable actuating lever in order to open or close the weighted control valve, it is not always assured that the control arrangement will operate with the requisite reliability. The known control arrangement is capable of actuating either one control valve or a second control valve disposed downstream of it; these valves jointly trigger a single shutoff valve.